The invention relates to an arrangement for determining the heart rate or refractory time of the cardiac tissue having an electrode for sensing heart action signals, an input stage connected to the electrode for processing the heart action signals, a refractory member for ascertaining a refractory time of the arrangement after a predetermined segment of a heart action segment, and a processing device connected to an output of the input stage for determining the rate of the heart action signals, processed by blanking components of a heart action signal that occur during the refractory time.
The frequency of the natural actions of the heart (heart rate) is of overwhelming significance in controlling heart rhythm correction devices—especially, implantable pacemakers in the treatment of bradyarrhythmias or tachyarrhythmias, but also of defibrillators and cardioversion devices. Correctly detecting this variable, especially the rate detected in the heart chamber (ventricular rate) has therefore been the subject of special development work, at least since the development of the demand-type pacemaker.
In context of this work, the known arrangements has risen for automatic gain control or adaptive threshold value processing of the heart signals used to determine the heart rate. According to known arrangements, the heart signals detect fluctuations in the heart signal amplitude (as the primary source of error in rate determination) and these fluctuations are to be compensated for.
Moreover, because of the special signal shape of the heart action signals, the use of refractory time members has become established in arrangements for determining the heart rate.
A typical heart action signal of a ventricular action, which is also known as the QRST complex, is shown in FIG. 1a: The problem that makes it expedient to use a refractory member is clearly shown in FIG. 1b in terms of two successive heart action signals: If the signals are subjected to threshold value processing, with a threshold value (symbolized by a dashed line designated “+Vt” or “−Vt” on either side of the zero signal line), or in other words if the signal components located above +Vt (or optionally below −Vt) are evaluated as a “heartbeat”, mismeasurements can occur in practice, especially if the maximum amplitude in the T portion of the QRST complex is above +Vt.
To prevent such a “T-wave” next to the so-called “R-wave” from being evaluated as a separate heartbeat—an effect generally called “oversensing”—the input stage is assigned a refractory member, in which the portions of the heart action signal that in FIG. 1b are located between the vertical dot-dashed lines—which indicate the boundaries of the refractory time, or the refractory interval “REF”—are blanked out. As can be seen from the example shown in the drawing, the TRR spacing of the R-waves is correctly evaluated as a heartbeat interval, thus fundamentally enabling the avoidance of oversensing.
From T. Parviainen et al, “Ratemeter based on analogue divider”, Med. & Biol. Eng. & Comput. 1978, 16, 121, a device for determining the heart rate is known in which optimally blanking out (“blocking”) of a time segment of 20 to 70% of the heartbeat interval is contemplated, to eliminate P-waves as an interference signal.
It is also known to program the refractory time patient-specifically and thus largely to take appropriate account of the individual heart signal shape.
Especially in certain tachyarrhythmias and in transition regions between tachycardic rhythm malfunctions that have a pronounced periodic (“sinusoidal”) heart signal course and heart fluttering (fibrillation), strong amplitude fluctuations between the heart signal complexes can occur in the relative amplitudes of the individual signal segments, and such heart rate fluctuations can all occur parallel to one another. Under such conditions, even with a patient-specifically programmed refractory time, oversensing or—the converse, the nondetection of R-waves (“undersensing”) can no longer be reliably precluded.